Field of the Invention
The present invention relates to an image pickup apparatus such as a digital camera and a digital single-reflex camera, digital video cameras, or a mobile terminal having an image pickup function, and a control method therefor.
Description of the Related Art
In image pickup apparatuses such as digital cameras, due to increase in magnification, image burring caused by camera shake or the like tends to be noticeable on a telephoto side where magnification is high, and further performance improvement of an image stabilization apparatus is demanded. The image stabilization apparatus detects camera shake or the like caused by photographer's movement, moves an image stabilization optical system in a direction substantially perpendicular to an optical axis so as to cancel the camera shake or the like caused by photographer's movement.
In order to meet the demand for performance improvement of the image stabilization apparatus, the image stabilization optical system needs to be moved to a large extent in the direction perpendicular to the optical axis, and this means that the image stabilization optical system goes farther away from the optical axis.
On the other hand, due to increase in the resolution of recent image pickup apparatuses such as digital cameras, even a subject being slightly out of focus is easily noticeable, and hence more accurate focusing is required. Examples of methods for focusing on a subject include a triangulation method that attains focus by measuring a distance to a subject using an installed external distance measurement sensor. Another method for focusing on a subject is a contrast AF method that automatically adjusts a focus using an AF evaluation value extracted by filtering a specific frequency component of a luminance signal obtained from an image pickup apparatus.
Further, in order to reduce effects of camera shake on focusing accuracy, there has been proposed a technique that changes a weight assigned to a first in-focus position obtained by the triangulation method and a weight assigned to a second in-focus position obtained by the contrast AF method, thus obtaining a third in-focus position (see Japanese Laid-Open Patent Publication (Kokai) No. 2009-145852).
However, as described above, if the image stabilization optical system is moved far from the optical axis, peripheral illumination and a subject contrast at the center of an image is decreased, and this brings about degradation in optical performance.
According to the AF contrast method, before the image pickup device is exposed to light, scanning is performed to obtain contrast levels of a subject (mountain obtainment scanning) while a focus lens is being moved over a predetermined section, and during shooting, the focus lens is moved to a position at which contrast is at its peak.
As a result, a subject is brought into focus, but when the image pickup apparatus is wobbled due to, for example, camera shake caused by photographer's movement while the mountain obtainment scanning is performed, the image pickup apparatus detects the wobble and moves the image stabilization apparatus in such a direction as to cancel the wobble. At this time, when the image stabilization optical system moves far from a center of the optical axis, a shape of a mountain found by the mountain obtainment scanning differs from that of a mountain found in a case where the image stabilization optical system lies at the center of the optical axis.
Here, camera shake caused by photographer's movement always occurs, and hence subject contrast as well always changes. For this reason, when the focus lens is moved to a position where contrast scanned by the mountain obtainment scanning immediately before shooting lies at its peak, the focus lens may be off an in-focus position.
Moreover, although during exposure, the focus lens is fixedly held without being moved, the image pickup apparatus detects a wobble such as camera shake caused by photographer's movement or the like and moves the image stabilization optical system even during exposure as well, and therefore, an exposure is made at a position where subject contrast is low.